Lām is used as a prefix in two different ways. Lām-kasra (لـِ, /li/) is essentially a preposition meaning "to" or "for", as in لِوالديliwālidī, "for my father". In this usage, it has become concatenated with other words to form new constructions often treated as independent words: for instance, لِماذاlimāḏā, meaning "why?", is derived from لـِ li and ماذاmāḏā, meaning "what?" thus getting "for what?". This construction is virtually semantically identical the equivalent in most Romance languages, e.g. Frenchpourquoi, Spanishpor qué, and Italianperché (though ché is an archaism and not in current use).

The other construction, lām-fatḥa (لَـ/la/) is used as an emphatic particle in very formal Arabic and in certain fixed constructions, such as لَقدlaqad (itself an emphatic particle for past-tense verbs) and in the conditional structure لو...لَـlaw...la, effectively one of the forms of if...then....